1. Field of the Invention
The present invention relates to recording and reproducing devices, in particular, a recording and reproducing device for recording holographic information with two luminous fluxes of reference light and information light, and for reproducing the holographic information using reference light.
2. Description of the Related Art
A holographic recording method is proposed as a method for multiplexedly recording information on the same position.
Holographic recording is recording performed by separating a luminous flux from the same light source into reference light and information light; irradiating the same position on a recording medium with the reference light and the information light; generating a different interference by changing an irradiation angle or a wavelength of the reference light; and overlaying different pieces of information on the same position on the medium (see, e.g., Japanese Unexamined Patent Publication No. 2004-158114).
Herein, information light modulated into two-dimensional page information is obtained by arranging a spatial light modulator on a light path of the information light and transmitting or reflecting the luminous flux in a unit of pixels of the spatial light modulator.
The reference light, on the other hand, passes through a route different from one through which the information light passes, and is overlaid on the information light on the recording medium, so that an interference pattern is formed on the medium.
When reproducing the information recorded on the medium, only the reference light is applied onto the recorded region of the medium.
When the medium is irradiated with the reference light, reproduction light is generated, and a two-dimensional page light detector detects such reproduction light.
A reproducing signal, i.e., a light and dark signal corresponding to the recorded two-dimensional page information is output from the two-dimensional page light detector, and the recorded information is read by decoding the reproducing signal.
A CCD (Charge Coupled Device), a CMOS sensor or the like used in a digital camera and the like is used as the two-dimensional page light detector.
When reproducing the two-dimensional information of, for example, 1024 bits×1024 bits (=approximately one million bits), the CCD having an element of 1024 bits×1024 pixels (picture elements) is used.
In the CCD used in the digital camera and the like, even if one element of the CCD is defective, a rapid light intensity change is small near the defective pixel; therefore, an image to be output is usually recognized as a normal image as a whole to human eyes.
In devices having a one-dimensional solid state image sensor such as a fax machine, there is no situation in which an image to be output is entirely unreadable even when one pixel of the solid state image sensor is defective.
In such devices having the above image sensor, the defective pixel position is recorded in a memory in advance, and reproducing data of the defective position is corrected using data of normal pixels around the defective position to enhance image quality (see, e.g., Japanese Unexamined Patent Publication No. Sho 62(1987)-86968).
This correction is for generating data that is an approximate of the original data, and is not for reproducing the original data itself.
When recording information such as document data as the holographic data, the entire document data may not be reproduced if a defect is found in only one pixel. Further, when the information is not an image, there may be a rapid change in the light intensity between adjacent pixels, and an effective correction may not be performed on the defective pixel. Therefore, in the CCD used in the two-dimensional page light detector, all elements are required to operate properly. That is, it is necessary to manufacture a recording and reproducing device that uses only a CCD in which all elements are normal and are defect free.
However, in reality, when manufacturing a CCD having at least a few million pixels as used herein, it is difficult to constantly manufacture a non-defective CCD.
Further, a sophisticated manufacturing technique is necessary in manufacturing the non-defective CCD at good yield, leading to increase in cost.
As the number of pixels increase, the probability of manufacturing the defective CCD increases, thereby lowering the yield.
Thus, from the point of balance between the manufacturing cost and performance, in reality, a recording and reproducing device that has no problems in terms of practical use is desirably shipped even if a defective pixel that is not operating properly is present at a part of the CCD. Likewise, even if some defects are present in the optical components relating to reproduction other than the CCD and in optical components relating to recordation such as the spatial light modulator, it is desirable that the recording and reproducing device perform recording and reproducing without any problem.